The semiconductor pressure transducer has acheived widespread use in a variety of applications involving the medical, aerospace and automotive industries. The high output obtainable from semiconductor transducers employing piezoresistive sensing elements have specified their use in a wide range of applications and structures. These factors together with constant improvement in integrated circuit technology, have allowed the design of ultraminiature transducers which are capable of operating in very diverse environments.
It is, of course, known that a major problem in regard to these devices is the problem of compensating the transducer for changes both in temperature and pressure. As is known, the gage factor and resistivity of the sensor varies with temperature. In order to construct a transducer whose output and zero unbalance are essentially independent of temperature, the temperature coefficient of gage factor and resistance should be maintained as low as possible. In practice, it is extremely difficult if not virtually impossible to maintain a gage factor independent of temperature. Hence, the prior art transducers employ compensating techniques which are integrally associated with the sensor configuration. A usual method for output compensation utilizes the increase of bridge resistance to offset the decrease of gage factor with temperature. Hence, in such applications, it was important that the temperature coefficient of resistance over the entire range of interest be positive and somewhat larger than the decrease of gage factor with temperature.
In order to compensate for such effects, many circuits and techniques were devised to add external resistors to the bridge circuit in order to control the output of the bridge according to temperature. Typical techniques employed a plurality of resistive elements which, if properly arranged, would provide a temperature compensated transducer. See, for example a patent entitled TEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNIT, U.S. Pat. No. 3,245,252 issued on Apr. 12, 1966 to David J. First, Anthony D. Kurtz and Jean-Pierre A. Pugnaire. In any event, typical techniques to compensate strain gage transducers may require the addition of eleven or more external resistors to provide temperature compensation.
It is thus apparent that apart from the difficulty in compensating such structures, is the further difficulty that the components used to compensate the bridge characteristics can also vary and change according to temperature or with time. Hence, such devices are continuously monitored to assure proper calibration with temperature.
Apart from the temperature problems as described above, is the further problem that the semiconductor transducer or bridge configuration further exhibits nonlinearities due to applied pressure. Hence, even under the conditions of constant temperature, there is a variation in output due to pressure. This variation is in part due to the particular construction of the pressure responsive diaphragm as well as the construction techniques for fabricating the individual sensors. This variation of the output of the transducer with respect to pressure, as indicated, is not related to the temperature changes and does not vary according to the same relationship as governing the changes in temperature. Hence, the manufacturer must further account for such variations in output which are due strictly to the variation in pressure.
There are also techniques which employ compensating resistors in conjunction with temperature compensating components to attempt to compensate for the pressure effect.
Essentially, the problem in transducer output of pressure is that a transducer should ideally provide a linear voltage for applied pressure. For example, if a transducer provides one volt for one psi pressure, it should provide a half a volt for one-half psi pressure. In practice, the units do not do so and exhibit a definite variation according to applied pressure. Thus, as above indicated, this nonlinearity or variation is further compensated for by the use of external components and involves additional time and expense in production.
The problem of compensating a transducer is, of course, initially imposed upon the manufacturer of such devices who must assure that the units will operate according to specifications.
Hence, by using the above described techniques, the units produced are individually compensated during production. These techniques require an extensive amount of time and are implemented by relatively skilled workers. Such considerations, of course, are indicative of the price at which extremely accurate and highly calibrated pressure transducers are sold at.
It is therefore an object of the present invention to provide an improved pressure transducing structure employing digital circuitry capable of automatically and continuously compensating for temperature and pressure variations in such transducers. The techniques and structure to be described are applicable in general to provide automatic compensation for such transducing devices to thereby substantially reduce the above described production procedures presently employed in the transducer art.